A pacemaker is a medical device for implant within a patient, which recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmia. An ICD is a device, also for implant within a patient, which additionally or alternatively recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate the fibrillation. Herein, the term “cardioversion” refers to the delivery of shocking pulses intended to defibrillate the atria. “Defibrillation” refers to the delivery of shocking pulses intended to defibrillate the ventricles.
An exemplary ICD 10 along with various sensing/pacing/shocking leads are shown in FIG. 1. To sense atrial cardiac signals and to provide right atrial chamber pacing therapy, the ICD is coupled to an implantable right atrial lead 20 having an atrial tip electrode 22 and an atrial ring electrode 23. To sense left atrial and ventricular cardiac signals and to provide left chamber pacing therapy, the ICD is coupled to a “coronary sinus” lead 24 designed to receive atrial and ventricular cardiac signals and to deliver: left ventricular pacing therapy via a left ventricular tip electrode 26; left atrial pacing therapy via a left atrial ring electrode 27; and cardioversion shocks via a left atrial coil electrode 28. A right ventricular lead 30 includes a right ventricular tip electrode 32, a right ventricular ring electrode 34, a right ventricular (RV) coil electrode 36 and a superior vena cava (SVC) coil electrode 38. The right ventricular lead is capable of receiving cardiac signals and delivering pacing stimulation via the tip and ring electrodes 32 and 34 and delivering defibrillation shocks via coil 36 and SVC coil electrode 38. Cardioversion shocks are typically delivered with up to 10 joules of energy. Defibrillation shocks are much stronger and typically employ up to 40 or more joules of energy. Both cardioversion shocks and defibrillation shocks are delivered at high voltages, on the order of 800 volts. ICDs are typically designed to be capable of delivering at least a hundred defibrillation shocks.
To withstand the high voltages and large quantities of energy required for cardioversion and defibrillation shocks and to permit delivery of a large number of shocks over the lifetime of the device, ICDs and their leads are designed as robustly as possible. To this end, ICDs typically include one or more silver vanadium oxide (SVO) batteries along with one or more large capacitors, each formed of aluminum oxide (Al2O3). The SVO batteries provide power for all cardioversion and defibrillation shocks as well as for all pacing and monitoring functions. Once the ICD is implanted, it is not intended to be removed, hence the SVO power source used therein must be sufficiently large to provide power to deliver a hundred or more of defibrillation shocks, as well as to provide power for long-term pacing and sensing functions, typically for the lifetime of the patient. In the rare case where the power cell becomes depleted, the ICD is replaced with another similar ICD.
The SVO batteries and the aluminum oxide capacitors are quite large in size and weight and, to ensure that the capacitors can be promptly be charged to high-voltages upon detection of defibrillation, the capacitors are periodically reformed, i.e. the capacitors are charged to capacity every three months or so, even if no shock is required. By periodically reforming the aluminum oxide capacitor, ten seconds or more can be reduced in charge time as compared to a capacitor that has not been reformed, thus allowing an initial defibrillation pulse to be delivered as quickly as possibly upon detection of an episode of ventricular fibrillation. Charge delivered to the capacitor during reformation is dumped from the capacitor through a dump resistor provided within the implanted device so that the energy may be dissipated as heat.
As noted, the SVC coil, the right ventricular coil, and the right ventricular tip and ring electrodes are all provided within a single right ventricular lead (lead 30 of FIG. 1). Because the lead must accommodate the currents and voltages required for numerous cardioversion and defibrillation shocks, the lead typically employs electrical insulation along its entire length. The insulation is provided, in part, to ensure that defibrillation shocks delivered to the right ventricular coil are not short-circuited.
Additionally, cut-off switches are provided within the ICD to help prevent shocking currents from being conducted back through the leads and into the device electronics, which might damage or disable the device electronics. In particular, cut-off switches are provided on all pacing outputs (tip and ring) leads. The switches also help prevent tissue necrosis from occurring within myocardial tissue near the electrodes as a result of “in rush” current. For example, if cut-off switches were not provided, current delivered by the right ventricular coil electrode would likely pass through myocardial tissue and into the right ventricular tip electrode. Current densities near the tip electrode can damage myocardial tissue, particularly with delivery of numerous shocks. In this regard, within most ICDs, the right ventricular tip and ring electrodes are held at voltages intermediate the voltage of the device case and the voltage of the right ventricular coil. Typically, a voltage difference between the case and the right ventricular coil upon delivery of a defibrillation pulse is 800 volts. Hence, there is usually about a 400 volt difference between the device case and the right ventricular tip and ring electrodes and another 400 volt difference between the right ventricular tip and ring electrodes and the right ventricular coil electrode. Similar voltage differences may occur between the atrial tip and ring electrodes and the device case and the right ventricular coil. These large voltage differences necessitate the cut-off switches. However, such switches are bulky and consume considerable space within the ICD housing. In addition to the switches themselves, additional circuitry and microprocessor logic must be provided to coordinate the use of the switches and, should one of the switches fail, the device electronics are in jeopardy.
The aforementioned features are illustrated in the diagram of FIG. 2. Briefly, a single SVO power source 51 provides all power for the device including power to operate control circuitry 52, which controls all pacing, monitoring, capacitor reformation, cardioversion and defibrillation control functions. The single power supply also provides power for delivering pacing pulses through right atrial, left atrial and right ventricular tip and ring electrodes as well as to provide biphasic cardioversion and the defibrillation shocks via the left and right ventricular coil electrodes, the left atrial coil, and the SVC electrode. Exemplary electrode voltages are listed in FIG. 2. These are merely provided to illustrate voltage differences—actual absolute voltages may differ.
During pacing, switches 53, provided along the pacing/sensing leads, are closed to allow pacing and sensing. For cardioversion or defibrillation, a capacitor charging switch 54 is toggled at high frequencies to begin charging an aluminum oxide capacitor 55 via a voltage transformer 56. For cardioversion, up to ten joules are stored within the capacitor. Just prior to delivery of the cardioversion shock, switches 53 are opened to prevent current from propagating into control circuitry to via the pacing and sensing leads. Then appropriate switches within shock switching circuitry 57 are closed to route the energy stored with capacitor into the atria as a biphasic shocking pulse using, for example, the AL coil and the case as electrodes. For defibrillation, about 40 joules of energy are stored within the capacitor and then appropriate switches within switching circuitry 57 are closed to route the energy into the ventricles as a biphasic pulse using the right ventricular coil electrode in combination with either the left ventricular coil, the SVC or the case. In early devices, defibrillation shocks were usually delivered between the RV coil and the SVC coil. More recent devices typically deliver the defibrillation shock between the ventricular coil electrodes and the device can. The RV-SVC coil delivery is still employed in some patients, as it is believed that a lower defibrillation threshold can be achieved, i.e. less energy may be required within the shock to defibrillate the heart. In nay case, once a cardioversion or defibrillation shock is delivered, switches 53 are again closed to allow sensing to determine whether the shock was successful and, if not, switches 53 are again opened and additional cardioversion or defibrillation shocks are delivered to the heart of the patient.
If no shocks are delivered for about three months, control circuitry 52 initiates a capacitor reformation process wherein capacitor 55 is charged to capacity, without delivering a shock to the patient. After reformation, a dump resistor switch 58 maybe closed to allow the charge of the capacitor to be dissipated through a dump resistor 59. The dump resistor is also used if charge stored on the capacitor exceeds the programmed therapy value. The capacitor reformation process is summarized in FIG. 3. Briefly, a capacitor reformation timer is initially set at step 60, typically to time a three-month period. Then, beginning at step 61, the ICD monitors cardiac rhythm to deliver pacing therapy and to detect atrial or ventricular fibrillation and, if fibrillation is detected, the internal capacitor is charged and an appropriate shock is delivered, at step 62. Once a shock is delivered, the capacitor reformation timer is then reset at step 60 since the process of charging the capacitor to deliver the shock serves to reform the capacitor. The device then continues to monitor for atrial or ventricular fibrillation, at step 61. So long as no fibrillation is detected, the device simply continues delivering any needed pacing therapy and monitoring for fibrillation. Eventually, if no shocks are delivered, the timer expires and, at step 63, the ICD performs the capacitor reformation procedure wherein the capacitor is charged to its maximum capacity. Note, however, that dumping is not a requirement. Alternatively, the charge can be allowed to dissipate internally via leakage. In any case, the reformation timer is reset at step 60. As noted, by periodically reforming the capacitor if no shocks are delivered, the capacitor can be more quickly charged to capacity when a shock is required.
As can be appreciated, the need to provide the aforementioned ICD features and lead features for accommodating the large currents and voltages required for defibrillation results in an ICD and lead assembly that is large, heavy, complex and quite expensive. Hence, ICDs are typically implanted only within the patients who are at a significant risk of ventricular fibrillation, such as patients who have had a previous myocardial infarction. More specifically, ICDs are usually only implanted within patients classified as Class I patients within the ICD implantation guidelines of Table I.
TABLE 1Class I1.Cardiac arrest due to VF or VT not due to a transient orreversible cause. (Level of evidence: A)2.Spontaneous sustained VT. (Level of evidence: B)3.Syncope of undetermined origin with clinically relevant,hemodynamically significant sustained T or VF induced atelectrophysiological study when drug therapy is ineffec-tive, not tolerated, or not preferred. (Level ofevidence: B)4.Nonsustained VT with coronary disease, prior MI, LV dys-function, and inducible VF or sustained VT at electro-physiological study that is not suppressible by a Class Iantiarrhythmic drug. (Level of evidence: B)Class IIaNoneClass IIb1.Cardiac arrest presumed to be due to VF when electro-physiological testing is precluded by other medicalconditions. (Level of evidence: C)2.Severe symptoms attributable to sustained ventriculartachyarrhythmias while awaiting cardiac trans-plantation. (Level of evidence: C)3.Familial or inherited conditions with a high risk forlife-threatening ventricular tachyarrhythmias such aslong QT syndrome or hypertrophic cardiomyopathy.(Level of evidence: B)4.Nonsustained VT with coronary artery disease, prior MI,and LV dysfunction, and inducible sustained VT or VF atelectrophysiological study. (Level of evidence: B)5.Recurrent syncope of undetermined etiology in thepresence of ventricular dysfunction and inducibleventricular arrhythmias at electrophysiologicalstudy when other causes of syncope have beenexcluded. (Level of evidence: C)Class III1.Syncope of undetermined cause in a patient without inducibleventricular tachyarrhythmias. (Level of evidence: C)2.Incessant VT or VF. (Level of evidence: C)3.VF or VT resulting from arrhythmias amenable to surgical orcatheter ablation; for example, atrial arrhythmias associatedwith the Wolff-Parkinson-White syndrome, right ventricularoutflow tract VT, idiopathic left ventricular tachycardia orfascicular Vt. (Level of evidence: C)4.Ventricular tachyarrhythmias due to a transient or reversibledisorder (e.g., AMI, electrolyte imbalance, drugs, trauma).(Level of evidence: C)5.Significant psychiatric illnesses that may be aggravated bydevice implantation or may preclude systematic follow-up.(Level of evidence: C)6.Terminal illnesses with projected life expectancy </=6 months. (Level of evidence: C)7.Patients with coronary artery disease with LV dysfunction andprolonged QRS duration in the absence of spontaneous orinducible sustained or nonsustained VT who are undergoingcoronary bypass surgery. (Level of evidence: B)8.Class IV drug-refractory congestive heart failure in patientswho are not candidates for cardiac transplantation.(Level of evidence: C)Level of evidence: Level A = well-designed, controlled clinical trials; B = cohort studies; C = expert opinion
Moreover, to maximize the lifetime of the SVO power source, the ICD must be programmed to deliver shocks with the lowest magnitude sufficient to reliably defibrillate the ventricles (plus some safety margin). Accordingly, the physician usually must perform a ventricular fibrillation induction test wherein the physician triggers ventricular fibrillation within the patient then controls the ICD to deliver various shocks of differing magnitudes so as to determine the minimum shocking energy sufficient to reliable defibrillate the patient. The fibrillation induction procedure is summarized by FIG. 4. Briefly, prior to implant of an ICD, a determination is made as to whether the patient is at high risk of ventricular fibrillation, i.e. a determination is made as to whether the patient is classified within the Class 1. Assuming class 1, then an ICD, such as the one illustrated in FIGS. 1 and 2, is implanted within the patient, along with leads capable of providing for pacing, cardioversion and defibrillation. At step 72, the physician sets the shock magnitude for defibrillation shocks to a low test magnitude then, at step 73, induces ventricular fibrillation within the heart of the patient. At step 74, the implanted ICD detects the fibrillation and delivers a test shock at the selected shock magnitude. If the shock does not successfully defibrillate the heart, then the ICD increases the shock magnitude, at step 75, and another shock is delivered. Once a shock successfully defibrillates the heart, then, at step 76, a safety margin is added to the shock magnitude that proved successful and, at step 77, the patient is sent home with the ICD set to deliver pacing therapy, if needed, and to respond to potentially numerous episodes of atrial or ventricular fibrillation, should such episodes arise. In the example FIG. 4, the induction test is performed to set the shock magnitude by selectively increasing from a minimum shock magnitude. In other examples, the test begins with a maximum shock magnitude, which is incrementally decreased. Still other specific approaches can be employed. The intentional induction of ventricular fibrillation within a patient is, of course, a risky procedure and hence many physicians are reluctant to perform the procedure and many patients are reluctant to undergo the procedure. Within patients for whom induction testing is performed, the associated costs of the overall implantation of the ICD are significant increased. This is another reason why ICDs are typically only implanted within patients at significant risk of ventricular fibrillation.
Despite the significant costs, an ICD can be lifesaving and hence ICDs are now commonly implanted in patients who are at a high risk of ventricular fibrillation, i.e. Class 1 patients. However, ventricular fibrillation sometimes also arises within patients who do not appear to be at significant risk (i.e. non-Class 1 patients), including the patients who would otherwise receive only a conventional pacemaker. Within such patients, should ventricular fibrillation nevertheless occur, it often proves fatal because there is typically not sufficient time for paramedics to arrive with an external defibrillator to defibrillate the heart.
Accordingly, it would be highly desirable to provide an implantable medical device capable of delivering defibrillation therapy for implantation in patients not perceived to be at high risk of ventricular fibrillation (i.e. for use in non-Class 1 patients.) Such a device would not be as bulky, heavy, and expensive as a “full-service” ICD of the type described above and would not require fibrillation induction testing.